1. Field of the Invention
This invention relates to laser shock peening of metallic objects such as gas turbine engine parts and, more particularly, to coating laser shock peening surfaces of a workpiece with an ablative material containing one or more explosive ingredients.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. U.S. Pat. No. 5,674,329, entitled xe2x80x9cAdhesive Tape Covered Laser Shock Peeningxe2x80x9d and U.S. Pat. No. 5,674,328, entitled xe2x80x9cDry Tape Covered Laser Shock Peeningxe2x80x9d disclose the use of tape having an ablative layer as the ablative coating. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the workpiece or a transparent coating such as a clear plastic layer and ablative layer of a tape placed over workpiece. The curtain of water and clear tape layer provides a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP""D to create the beneficial compressive residual stresses. This water or other fluid confining medium also serves as a carrier to remove process generated debris and any unused laser beam energy. U.S. Pat. No. 5,932,120 entitled xe2x80x9cLaser Shock Peening Using Low Energy Laserxe2x80x9d discloses the use of low power lasers and laser beams which are less expensive to procure and use than the higher power lasers disclosed previously.
Laser shock peening is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
A method of laser shock peening a metallic part by firing a laser on a coated laser shock peening surface of the part which has been coated with a material having at least one explosive ingredient. Two or more explosive ingredients may be used wherein each of the explosive ingredients has a different shock sensitivity and the laser beam is fired with sufficient power to vaporize the coating and explode at least some amount of each of the explosive ingredients.
One embodiment of the invention includes forming an explosive coated surface by coating a laser shock peening surface on the workpiece with a material containing at least one explosive ingredient, continuously firing a laser beam which repeatably pulses between relatively constant periods, on the coated surface of the workpiece while providing continuous movement between the laser beam and the metallic workpiece, and firing the laser beam with sufficient power to explode at least some of the explosive ingredient with the pulses and forming laser beam spots on the coating and forming a region in the workpiece having deep compressive residual stresses imparted by the laser beam pulsing such that the region extends into the workpiece from the laser shock peening surface. Another embodiment of the invention includes forming an ablative coated surface by coating a laser shock peening surface on the workpiece with an ablative material containing at least one explosive ingredient, continuously firing a laser beam which repeatably pulses between relatively constant periods, on the coated surface of the workpiece while providing continuous movement between the laser beam and the metallic workpiece, and firing the laser beam with sufficient power to vaporize the ablative medium of the coating and to explode at least some of the explosive ingredient with the pulses and forming laser beam spots on the coating and forming a region in the workpiece having deep compressive residual stresses imparted by the laser beam pulsing such that the region extends into the workpiece from the laser shock peening surface. Various embodiments of the invention includes flowing a fluid curtain, while firing the laser beam over the coated surface upon which the laser beam is firing to form a pattern of overlapping laser beam spots, while moving the laser relative to the workpiece. In alternate embodiments, the coating material contains two or more explosive ingredients wherein each of the explosive ingredients has a different shock sensitivity and the laser beam is fired with sufficient power to explode at least some amount of each of the explosive ingredients. Suitable explosive ingredients nitroglycerin, pentaerythritol tetranitrate (PETN), RDX, and ammonium nitrate.
The laser shock peening surface may be coated with a film using an adhesive or a self-adhering tape containing the explosive ingredients. The film or tape may include an ablative material. The ablative coated surface may be formed by coating the laser shock peening surface with a liquid coating with or an ablative material and containing the explosive ingredient(s) and then drying the coating on the laser shock peening surface. The laser shock peening surface may be coated with an ablative material and an oxidizer which acts as the explosive ingredient. The present invention enhances the explosive force of laser shock peening and, thus, can be used to lower the power of the laser, increase the size of the laser beam spot, and in turn, lower the cost, time, man power and complexity of laser shock peening.